Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/06—Alloys based on aluminium with magnesium as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/04—Making non-ferrous alloys by powder metallurgy
  • C22C1/0408—Light metal alloys
  • C22C1/0416—Aluminium-based alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C1/00—Making non-ferrous alloys
  • C22C1/10—Alloys containing non-metals
  • C22C1/1084—Alloys containing non-metals by mechanical alloying (blending, milling)
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C32/00—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
  • B22F2998/10—Processes characterised by the sequence of their steps

Definitions

• This invention relates to high strength isotropic dispersion strengthened aluminium-lithium alloys and in particular to those alloys suitable for fabrication, via an mechanically alloying route, into forged, extruded or rolled products made therefrom.
• Aluminium-lithium alloys are of interest in these fields because the addition of lithium offers the possibility of improving properties of aluminium with respect to density and elastic modulus.
• the level of addition of lithium is chosen to avoid precipitation of the ⁇ ' phase, Al 3 Li which would render the alloy heat treatable and so compromise its properties.
• Magnesium additions are also known to reduce the solubility of the lithium in the matrix and thus can render the alloy susceptible to age hardening. Magnesium has the further advantage of adding a component of solid solution strengthening. To meet the qualifications for certain advanced design applications, a combination of property requirements must be met including required density, strength, corrosion resistance, fracture toughness and ductility.
• Aluminium-lithium-magnesium alloys manufactured by the abovementioned methods which have increased strength have been disclosed in EP 0 180 144.
• Heat treatable aluminium-lithium alloys which do not suffer from a reduction in strength as a result of the heat treatment process have been disclosed in EP 0 194 700.
• US 4,600,556 discloses aluminium-lithium-magnesium alloys with improved strength and fracture toughness without an unacceptable loss in ductility. It has now been surprisingly found that high (4-6%) magnesium contents increase toughness and do not cause ⁇ ' phase precipitation provided the lithium content is less than 1.6%.
• aluminium alloys with reduced health and safety problems as well as improved mechanical properties particularly isotropic strength and fracture toughness.
• the ultimate product forms of these materials are often complex shapes and it is a further object of the invention to provide aluminium alloys which can be manufactured and shaped using cost effective techniques, whilst retaining their desirable properties.
• the present invention provides a method for the production of an aluminium-based alloy product comprising preparing a mechanically alloyed powder having a composition within the following ranges, all of the ranges being specified in weight percent: lithium 1.2 to 1.4 magnesium 4.5 to 5.5 carbon 0.25 to 0.40 oxygen up to 1.0, and optionally, up to 2.0 of grain controlling elements, said elements being selected from scandium, titanium, vanadium and niobium at up to 0.2 weight percent, nickel and chromium, at up to 0.5 weight percent and preferably at up to 0.2 weight percent, hafnium at up to 0.6 weight percent, and cerium at up to 0.5 weight percent,the balance being aluminium and incidental impurities; de-gassing and consolidating the powder at an elevated temperature followed by fabrication of the de-gassed and consolidated powder into the ultimate product form of the same composition.
• the principal alloying elements are lithium and magnesium with further optional additions of up to 2.0% of one or more of the elements selected from those established in the art as suitable for microstructural optimisation and control.
• These further grain controlling elements are selected from scandium, titanium, vanadium and niobium at up to 0.2%, nickel and chromium at up to 0.5% and preferably at up to 0.2%, hafnium at up to 0.6% and cerium at up to 0.5%.
• the carbon and oxygen in the alloy is generally provided by a process control agent added during the mechanical alloying process.
• the carbon level is lower than normally used for mechanically alloyed powders; but is sufficient to allow the production of the mechanically alloyed powder and has a number of advantages.
• the carbon and carbides in the system generally decorate the grain boundaries in the manufactured product, which consequently reduces the fracture toughness of the material. By reducing the level of carbon in the system, the inventors have determined that the amount present at grain boundaries is similarly reduced, resulting in a reduced presence of stress raisers. Crack propagation is therefore more difficult and fracture toughness is increased.
• the aluminium alloy contains by weight percent: 1.2 to 1.4% lithium; 4.5 to 5.5% magnesium; 0.25 to 0.35% carbon, and up to 1% oxygen. These levels of alloying additions give a good balance of properties. The properties of lithium and magnesium are such that the effects of solid solution strengthening produced by the magnesium addition are not significantly reduced by the level of lithium addition.
• Alloys according to the invention are also found to exhibit improved isotropic tensile performance, fracture toughness and corrosion resistance.
• the alloy is mechanically alloyed and the resulting powder degassed and compacted into billets.
• hot isostatic pressing HIP is an example.
• Billets can be fabricated into the ultimate product forms by extrusion, rolling, forging or other known methods. If complex parts are to be manufactured with little waste, a preferred manufacturing route to use is forging. Forging allows complex parts to be manufactured with near net shape resulting in very little material wastage and post manufacture working of the product is kept to a minimum. Two important parameters in the forging stage are forging temperature and the amount and type of reduction the billet encounters.
• the forging temperature is critical to the metallurgical structure of the alloy. If the forging temperature is too high, the grains in the alloy grow which reduces the strength of the final product thus, the advantages gained in producing a mechanically alloyed powder are reduced. A number of factors influence the temperature that the alloy reaches during forging, including the temperature of the die, the temperature of the billet before entering the die, the speed of forging, the amount of reduction in forging and the thickness of the final part. These factors influence not only the mechanical property differences in different parts but also within a part.
• a preferred embodiment of the invention is characterised in that the alloy is forged at a temperature within the range 250 to 450 °C. In a more preferred embodiment, the alloy billet is forged within the range 300 to 400°C.
• the amount and type of reduction used in the forging stage affects both the temperature of the forging and the mechanical properties of the product. Shear stresses produced in the billet during forging cause the breakdown of oxide boundaries present on the powder particles of the mechanically alloyed material. By breaking down these oxide boundaries, the forging process disperses the oxides in the material, so reducing the chance of large particles of oxides being present on the grain boundaries of the forged product. This in turn results in a product with improved mechanical properties.
• the amount and type of reduction used depends partly on the type of forging process used. In open die forging, a reduction ratio of greater than 8:1 is necessary to fully develop the ductility of the alloy. In die forging, where the work is more constrained, lower reduction ratios are sufficient.
• the following table shows two alloys according to the present invention showing constituents and mechanical properties. For all properties except fracture toughness, the values given are an average taken from different testing directions. The fracture toughness results are from the T-L direction.
• the aluminium alloys were prepared using mechanically alloying. The powders were compacted and the resulting billets forged at 300°C. COMPOSITION UTS (MPa) 0.2% Proof Stress (MPa) Strain to Failure (%) Fracture Toughness (Mpam 1 ⁇ 2 ) 1.3% Li, 5.2% Mg, 0.35%C 500.6 441.4 11.90 23.1 1.3%Li,5.2%Mg, 0.25%C 491.5 432.4 11.15 17.1

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Powder Metallurgy (AREA)
• Battery Electrode And Active Subsutance (AREA)
• Cell Electrode Carriers And Collectors (AREA)
• Manufacture And Refinement Of Metals (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)

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• 1998
  • 1998-09-03 GB GB9819083A patent/GB2341612A/en not_active Withdrawn
• 1999
  • 1999-09-02 CA CA002341260A patent/CA2341260A1/en not_active Abandoned
  • 1999-09-02 AT AT99943099T patent/ATE244317T1/de not_active IP Right Cessation
  • 1999-09-02 WO PCT/GB1999/002893 patent/WO2000014291A1/en not_active Application Discontinuation
  • 1999-09-02 AU AU56382/99A patent/AU760734B2/en not_active Ceased
  • 1999-09-02 KR KR1020017002751A patent/KR20010073098A/ko not_active Application Discontinuation
  • 1999-09-02 GB GB0103338A patent/GB2363389B/en not_active Expired - Fee Related
  • 1999-09-02 US US09/762,763 patent/US6485583B1/en not_active Expired - Fee Related
  • 1999-09-02 DE DE69909307T patent/DE69909307T2/de not_active Expired - Fee Related
  • 1999-09-02 EP EP99943099A patent/EP1114198B1/en not_active Expired - Lifetime
  • 1999-09-02 JP JP2000569030A patent/JP3903412B2/ja not_active Expired - Fee Related

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